This invention relates generally to laser sources and more particular to laser diodes that have narrow emission apertures of less than one micron in width. Such laser diodes have utility as optical pickup or retrieval of optically stored data on optical recording media.
Optical pickup heads in current optical data storage systems impose a limit on the achievable data storage density. A recognized approach to increase the data storage density is to replace the conventional head with a laser diode held in close proximity with the storage medium so that the bit size is commensurate with the extent of the diode""ss near-field emission.
Various approaches are known to produce an optically switched laser (OSL) head for optical data storage systems. Two prominent approaches to achieve a low cost OSL head include the tapered laser adopted by researchers at NTT. See the published article of H. Ukita et al., SPIE, Vol. 1499, pp. 248-261 (1991) and U.S. Pat. No. 4,860,276 to Ukita et al. Also, improved optical mode confinement can be achieved in a laser diode if an aperture or recess for output beam emission is fabricated in the front facet coating as disclosed, for example, in U.S. Pat. No. 5,625,617 to Hopkins et al. Several other approaches exist and typically include the employment of a laser in combination with a solid immersion lens (SIL) or an integrated microlens, such as disclosed in the published article of Y. Katagiri et al., SPIE, Vol. 2514, pp. 100-111 (1995), or an integrated fiber probe using the near field, such as disclosed in U.S. Pat. No. 5,288,998. However, these approaches include additional components to the OSL head structure and increase the complexity of manufacturing as well as the cost of the optical head.
The patent to Berger et al., U.S. Pat. No. 5,208,821, discloses a laser diode formed by MOCVD growth over a patterned substrate comprising dovetailed mesas for forming a xe2x80x9cpinch-offxe2x80x9d active region of about 2 xcexcm to 4 xcexcm wide, as measured relative to the device window 21 formed in SiO2 layer 20. Similar dovetailed structures are shown in Japanese Laid Open Application No. 1-293687, published Nov. 27, 1989 and Japanese Laid Open Application No. 2-119285, published May 7, 1990. These structures, however, are not submicron-aperture laser diodes designed for improving recording or pickup density and threshold operation in optical data storage systems.
The taper laser structure of Ukita et al. In U.S. Pat. No. 4,860,276 is integrated on a substrate with a photodetector at the back facet used to monitor the state of the laser. The taper is introduced via two etched grooves on either side of the laser stripe that converge towards the emission facet and, as such, define the lateral mode confinement at the facet. The primary drawback of this approach is the accurate pattern alignment and high resolution photolithography required to define the mask layer for performing the etching of the trenches. Additionally, the minimum aperture size that has been demonstrated is 1 xcexcm. However, for providing enhanced density employing near field emission, a 1 xcexcm aperture is not small enough for efficient near field emission use. The lasers with apodization in the facet coating, demonstrated by Hopkins et al. in U.S. Pat. No. 5,625,617, may be derived from standard single mode lasers. However, to achieve submicron aperture size, the facet of each laser produced requires the formation of a hole in the facet coating created by focused ion beam (FIB) etching, which does not readily lend itself to high yields and standardized reproducibility.
Buried heterostructure lasers have been fabricated in GaAs/AlGaAs based material systems, as disclosed in the articles of E. Kapon et al., xe2x80x9cSingle Quantum Wire Semiconductor Lasersxe2x80x9d, Applied Physics Letters, Vol. 55(26), pp. 2715-2717 (1989); H. Narui et al., xe2x80x9cA Submilliampere-Threshold Multiquantum-Well AlGaAs Laser Without Facet Coating Using Single-Step MOCVDxe2x80x9d, IEEE Journal of Quantum Electronics, Vol. 28(1), pp. 4-8 (1992); and H. Zhao et al., xe2x80x9cSubmilliampere Threshold Current InGaAs-GaAs-AIGaAs lasers and Laser Arrays Grown on Nonplanar Substratesxe2x80x9d, IEEE Journal of Quantum Electronics, Vol. 1(2), pp. 196-202 (1995). Buried heterostructure lasers have been fabricated in InP based material systems, as disclosed in the articles of K. Uomi et al., xe2x80x9cUltralow Threshold 2.3 xcexcm InGaAsP/InP Compressive-Strained Multiquantum-Well Monolithic Laser Array for Parallel High-Density Optical Interconnectsxe2x80x9d, IEEE Journal of Select Topics in Quantum Electronics, Vol. 1(2), pp. 203-209 (1995) and T. R. Chen et al., xe2x80x9cStrained Single Quantum Well InGaAs Lasers with a Threshold Current of 0.25 mAxe2x80x9d, Applied Physics Letters, Vol. 63(19), pp. 2621-2623 (1993).
H. Zhao et al. in IEEE Journal of Quantum Electronics, Vol. 1(2), pp. 196-202 (1995) demonstrated that through growth of a buried heterostructure laser on a non-planar substrate, lateral active regions less than 0.5 xcexcm can be achieved in a GaAs/AlGaAs material system leading to a lateral and vertical near field widths of 0.5 xcexcmxc3x970.5 xcexcm. The Zhao et al. structure is illustrated in FIG. 1. To achieve this type of xe2x80x9cpinch-offxe2x80x9d active region structure, 2 xcexcm to 3 xcexcm wide lines on 250 xcexcm centers were photolithographically patterned onto the semiconductor substrate followed by a chemical etch that terminates on the (111) planes of the material. The narrow, pinch-off active region is formed because of facet dependent growth rates of the epitaxial layers grown onto the nonplanar substrate.
For high speed data links, buried heterostructure lasers have been optimized for low threshold, e.g., less than 1 mA, with high external efficiency, e.g., up to 80%, to around 2 mW output power, but have been demonstrated to, operate in a single mode to output powers as high as 40 mW to 60 mW.
Similar buried heterostructure laser diodes have been demonstrated by others, such as demonstrated by E. Kapon et al. where the structure is formed over a trough as opposed to formation over a mesa in the nonplanar substrate. However, none of these structures have been able to provide a buried heterostructure laser diode having an submicron aperture less than 0.5 xcexcm, which is an object of this invention.
It is a further object of this invention to provide a laser diode formed on a nonplanar substrate that has a submicron emission aperture with optical emitting mode confinement at the output facet to provide for submicron beam emission, such as below about 0.45 xcexcm wide emission aperture.
It is another object of this invention to provide a laser diode with a submicron emission aperture for utilizing a near field OSL head to extend the present limit of data density in optical recording and readout media employed in data storage and retrieval apparatus.
According to this invention, a buried heterostructure (BH) laser diode source with a narrow active region is disclosed for use in close proximity with optically-addressed data storage media for read/write functionality in a relatively high data density format. The BH laser source is formed on a pregrooved or prepatterned substrate to form mesas upon which epitaxial layers are formed to form laser source active regions that have small emission apertures at the laser source facet output. Selective removal of semiconductor cladding material and replacement of this material with lower refractive index materials provides a way of obtaining further mode-size reduction at the output facet of the laser source. Each mesa has a top surface and adjacent sidewalls such that in the growth of the epitaxial layers above the active region doped with a first conductivity type, the above active region epitaxial layers depositing on the top surface deposit as a first conductivity type and depositing on said sidewalls deposit as a second conductivity type. This growth construction provides for a naturally formed p-n junction at the laser source active region and eliminates the need to perform a subsequent diffusion process to form such a junction. The optical cavities of the laser sources may be tapered so that die cleaving at a predetermined point along the length of the optical cavity will provide the desired emission aperture size at the laser source output facet.
To extend the limit of data density in optical data storage, a submicron-aperture laser diode is realized for extending the limit on data density in optical data storage media using the near-field of the submicron-aperture laser diode in a pickup head of an optical disk recording and readout system. Laser diode structures employed by others in the art for OSL heads, as discussed in the Background, utilize fabrication processes requiring submicron photolithography introducing a tightly confining taper structure, or require high precision serial processing on fabricated/yielded laser diodes in order to drill a hole in laser facet coatings. However, the submicron-aperture laser diode of this invention relies substantially on standard photolithography and other applied laser diode processing techniques well adapted in the laser diode manufacturing industry. Adopting such an approach enhances manufacturability, enhances yields, and provides a relative cost advantage in employing the submicron-aperture laser diode of this invention as a pickup head of an optical disk readout system.
The laser diode disclosed here was principally designed for the purpose of use in high speed optical data link for efficient, ultra-low threshold operation with small emission apertures, such as less than 1 xcexcm and scaleable down to less than 0.5 xcexcm, to provide high power density output at the facet, without requiring specialized manufacturing processes. The laser diodes of this invention differ from heterostructure laser diodes conventionally used in optical recording and readout in that the lateral aperture width of the laser diode active region is reduced by more than a factor of three to closely match the vertical aperture width of the laser diode active region. An aperture size as small as 0.4 xcexcmxc3x970.4 xcexcm can be achieved, which aperture size is constrained by the index step that can be achieved in the semiconductor material system. Further reduction in near-field aperture width down to around 0.4 xcexcm can be achieved by improved core confinement in the vicinity of the output aperture by, for example, replacing the semiconductor cladding material adjacent to the active region with dielectric material.